Role of serotonin transporter and receptor gene variations in the acute effects of MDMA in healthy subjects

Vizeli and colleagues situate their study in the context of growing interest in MDMA (ecstasy, molly) both as a recreational drug and as an adjunct to psychotherapy, particularly for PTSD. MDMA's typical acute effects are largely mediated by the serotonin (5-HT) system through actions on tryptophan hydroxylase (TPH), the serotonin transporter (SERT, encoded by SLC6A4), and 5-HT receptor subtypes (5-HT1A, 5-HT1B, 5-HT2A). Previous pharmacological manipulations of these targets altered MDMA effects, and genetic variants in metabolising enzymes (notably CYP2D6) have been shown to affect MDMA pharmacokinetics and some pharmacodynamics. However, whether common genetic polymorphisms in genes encoding core 5-HT system components modify the acute subjective, empathic, physiological, or adverse responses to MDMA in humans remained uncertain, with only small or inconsistent findings reported in earlier studies. This pooled analysis tested whether common variants in selected serotonergic genes influence the acute response to a single oral 125 mg dose of MDMA in healthy subjects. Specifically, the investigators evaluated TPH1 (rs1800532, rs1799913), TPH2 (rs7305115), HTR1A (rs6295), HTR1B (rs6296), HTR2A (rs6313), and SLC6A4 5-HTTLPR together with an associated SNP (text refers to rs25531 and also to rs25331 in one place; the extraction does not clearly resolve this discrepancy). The primary aim was to determine whether these polymorphisms modulate MDMA-induced subjective, emotional, empathic, cardiovascular, thermogenic, and adverse effects, and to attempt replication of findings reported in smaller earlier studies.

This report is a pooled analysis of eight double-blind, placebo-controlled, crossover studies conducted by the same laboratory. The combined set originally included 136 healthy, mostly MDMA-naive volunteers of European descent aged 18–44 years; after exclusions for missing genotyping consent/samples and duplicate participations the final analytic sample comprised 124 subjects (64 women). Washout periods between single-dose administrations were at least 7 days. Only sessions with MDMA alone and placebo were used for this analysis. Participants received a single oral 125 mg dose of (±)MDMA hydrochloride in the fasting state between 08:00 and 09:00, followed by a small standardised lunch at 12:00–13:00. Doses were not adjusted for body weight or sex (mean dose 1.9 ± 0.3 mg/kg). Exclusion criteria included psychiatric or physical illness, and substantial prior illicit drug exposure (more than five lifetime uses), with urine tests used to exclude recent use. Sessions took place in a quiet hospital research ward with low physical activity; subjects were mostly recumbent and often listened to music. Outcomes comprised a broad set of measures sensitive to MDMA. Subjective effects were assessed repeatedly using Visual Analogue Scales (VAS) and the 60-item Adjective Mood Rating Scale (AMRS) at multiple time points up to 6 h post-dose. Empathy and emotion recognition were assessed using the Multifaceted Empathy Test (MET) and the Facial Emotion Recognition Task (FERT) at 90–180 min after dosing; MET and FERT data were available from 68 subjects. Physiological measures (systolic/diastolic blood pressure, heart rate, mean arterial pressure [MAP], rate pressure product [RPP], and tympanic temperature) were recorded at baseline and multiple time points up to 6 h. Acute and subacute adverse effects were measured with a 66-item List of Complaints (LC) administered at 3–6 h and 24 h post-dose. Plasma MDMA concentrations (Cmax and AUC0–6 h) were measured and reported. Genetic analyses tested the listed serotonergic polymorphisms (TPH1 rs1800532 and rs1799913, TPH2 rs7305115, HTR1A rs6295, HTR1B rs6296, HTR2A rs6313, and SLC6A4 5-HTTLPR with an associated SNP reported inconsistently as rs25531/rs25331 in the extraction). The genotype distributions were consistent with Caucasian reference cohorts. For repeatedly measured endpoints, peak effects (Emax) and area under the effect–time curve (AUEC, 0–6 h) were computed; MDMA–placebo differences (ΔEmax, ΔAUEC) were analysed using one-way ANOVA with genotype as the between-group factor. The area under the MDMA plasma concentration–time curve (AUC0–6 h) was included as a covariate to account for pharmacokinetic differences. The primary genetic model was additive; recessive or dominant models were examined when relevant. A Nyholt correction for multiple comparisons was applied across the many subjective, physiological, empathic, and adverse variables tested, yielding a corrected significance threshold of p < 0.00023 to control Type I error.

Overall, MDMA produced the expected acute subjective and physiological effects versus placebo. On VAS and AMRS measures, MDMA increased most ratings (except appetite, which decreased), and it significantly elevated blood pressure, MAP, RPP, and body temperature. Peak plasma MDMA concentrations in the 124 subjects averaged 226 ± 48 ng/ml, and mean AUC0–6 h was 954 ± 208 ng·h/ml. Genotype distributions matched previously reported Caucasian frequencies. In uncorrected analyses several genotype–effect associations emerged. Carriers of the HTR2A rs6313 A allele reported higher Emax ratings for “good drug effect,” “trust,” AMRS “well‑being,” “high‑mood,” and “dreaminess” compared with homozygous G carriers (F and p-values reported for each comparison, p-values < 0.05 to < 0.01). For SLC6A4 5-HTTLPR, carriers of a short allele showed higher ratings of “good drug effect,” “drug liking,” and “closeness to others,” and lower ratings of “bad drug effect,” compared with homozygous long-allele subjects; homozygous long (LALA) subjects showed higher AMRS “fear and depression” and lower “any drug effect,” “good drug effect,” and “drug liking” relative to certain other genotype groups. Regarding physiological outcomes, G-allele carriers of TPH2 rs7305115 had a higher peak body temperature than AA homozygotes (F1,121 = 4.84, p < 0.05). Adverse-effect analyses showed that TPH2 rs7305115 GG individuals reported more acute “lack of appetite” than AA individuals (means 0.76 ± 0.43 vs 0.36 ± 0.50; p = 0.017), and carriers of the A allele of HTR2A rs6313 reported less acute “dizziness” than GG homozygotes (means 0.25 ± 0.44 vs 0.55 ± 0.55; p = 0.0012). Plasma MDMA concentrations were similar across most genotypes, with two exceptions reported in the extraction: an association of rs6295 with MDMA AUC0–6 h (CG vs GG, p < 0.05) and rs6313 with MDMA Cmax. The investigators included plasma AUC as a covariate in genotype analyses to adjust for pharmacokinetic differences. Critically, after applying the Nyholt correction for multiple comparisons (corrected significance threshold p < 0.00023), none of the tested serotonergic gene polymorphisms significantly influenced MDMA-induced subjective, physiological, empathic, or adverse effects. Sex did not significantly modify genotype effects, and subgroup sample sizes for some assessments (for example MET and FERT; n = 68) were smaller than the full cohort.

Vizeli and colleagues interpret their findings as indicating that common genetic variations in several core serotonergic genes have, at most, a limited influence on the acute effects of a therapeutically relevant 125 mg MDMA dose in healthy young adults. Although uncorrected analyses suggested modest associations for TPH2 rs7305115, HTR2A rs6313, and SLC6A4 5-HTTLPR with some subjective, physiological, or adverse responses, these did not survive a stringent correction for multiple testing. The authors note that many of the polymorphisms investigated were assessed for the first time in the context of controlled MDMA administration in humans. The discussion places the null-corrected findings in relation to prior smaller studies that reported genotype associations, suggesting several possible explanations for discrepant results: earlier studies often used lower MDMA doses (75–100 mg) and did not always correct for multiple testing; genotype effects may be more evident at lower doses; and smaller samples are more vulnerable to chance findings. The authors also highlight methodological strengths of the present work — larger pooled sample, standardised laboratory conditions, and adjustment for plasma MDMA concentrations — while acknowledging limitations. Key limitations they report include the still modest sample size for detecting small genetic effect sizes, limited completion rates for some tasks (empathy and emotion recognition were done by only 68 subjects), restriction to mostly young healthy volunteers which limits generalisability to clinical or older populations, and the fact that not all potentially relevant genes or SNPs affecting MDMA action were tested. In conclusion, the study team suggests that interactions between MDMA and common 5-HT system genotypes are unlikely to be a major determinant of acute response when MDMA is used recreationally or in therapeutic settings, though they recommend replication in larger and more diverse samples to confirm these negative findings. The authors emphasise that identifying modulators of MDMA effects remains important given current therapeutic development, but their data indicate that the tested serotonergic polymorphisms do not markedly alter acute MDMA responses in healthy subjects.